It is proposed that cytological, histological and biochemical studies on tissues of insecticide-resistant populations of Gambusia affinis (mosquitofish) exposed to organochlorine compounds be conducted and compared with parallel studies of insecticide-susceptible populations and with controls. Information derived from these studies will be applicable to an understanding of the cellular adaptive processes of this vertebrate in response to chronic environmental stress, and towards prediction of changes taking place in other vertebrates. This understanding can lead toward the development of guidelines in planning invertebrate control programs. Specific parameters to be measured include lipid deposition in tissue, myelin redundancy, cytological effects of organochlorine compound exposure on nervous tissue, and endoplasmic reticular membrane and enzyme responses to organochlorine compound exposure. These studies could provide an understanding of the cellular basis of vertebrate insecticide resistance.